One of the most important solar events
from Earth’s perspective is
the coronal mass ejection (CME), the solar equivalent of a hurricane. A
CME
is the eruption of a huge bubble of plasma from the Sun’s outer
atmosphere, or corona. The corona is the gaseous region above the surface
that extends millions of miles into space. Thin and faint compared to the
Sun’ surface, the corona is only visible to the naked eye during a total
solar eclipse. Temperatures in this region exceed one million degrees
Celsius, 200 times hotter than the surface of the Sun.

How the corona can be so much hotter than the surface remains a mystery to
scientists, but most suspect that it has to do with the complicated
magnetic fields that burst from the interior and extend above the surface
in great arches and loops. The buildup and interaction of these magnetic
loops—which can stretch over, under, and around each other—seems to supply
the energy to heat the corona and produce the violent explosion of a CME.

According to some of the newest observations and theories, the larger and
higher magnetic loops of the Sun’s field are believed to hold down the
newer, smaller fields emerging from the surface. They also tie down the
hot plasma carried by those fields. Much like a net holding down a helium
balloon, this network of magnetic loops restrains the plasma and magnetic
fields trying to rise into the corona. This causes tremendous energy to
build. Eventually, some of the overlying magnetic loops merge and cancel
each other, cutting a hole in the magnetic net and allowing the CME to
escape at high speed.

Researchers compare this process to that of filling helium balloons.
If
you inflate a balloon without holding it down, it will slowly drift
upward. But if you hold the balloon down with a net, you can generate a
lot of force when you fill it, causing it to push upward. Once you remove
the net, the balloon shoots skyward.

Once it escapes the Sun’s gravity, a CME speeds across the gulf of
space
at velocities approaching one million miles per hour (400 km/sec), with
the fastest CMEs accelerating to 5 million mph. A typical CME can carry
more than 10 billion tons of plasma into the solar system, a mass equal to
that of 100,000 battleships. The energy in the bubble of solar plasma
packs a punch comparable to that of a hundred hurricanes combined.

Just hours after blowing into space, a CME cloud can grow to
dimensions
exceeding those of the Sun itself, often as wide as 30 million miles
across. As it ploughs into the solar wind, a CME can create a shock wave
that accelerates particles to dangerously high energies and speeds.
Behind that shock wave, the CME cloud flies through the solar system
bombarding planets, asteroids, and other objects with radiation and
plasma. If a CME erupts on the side of the Sun facing Earth, and if our
orbit intersects the path of that cloud, the results can be spectacular
and sometimes hazardous.